Some pathogens are spread from one person to another by direct contact. They leave the first person through body openings, mucous membranes, and skin wounds, and they enter the second person through similar channels. For example, the viruses that cause respiratory diseases such as influenza and the common cold are spread in moisture droplets when an infected person coughs or sneezes. A hand that was used to cover the mouth while coughing contains viruses that may be passed to doorknobs, so that the next person to touch the doorknob has a chance of picking up the infectious agent. The bacteria that cause some sexually transmitted infections, including gonorrhea and syphilis, are transmitted during sexual contact.

Other pathogens involve an intermediary carrier, such as an insect. The malarial parasite, for example, spends part of its life cycle in mosquitoes, then enters a person's bloodstream when the mosquito bites the person. Many pathogens are spread through contaminated food and water. Cholera bacteria, for example, are spread through food and water contaminated with the excrement of infected people.